1. Field of the Invention
The present invention relates to a length-measuring device for measuring, with a high degree of accuracy, space. intervals between the patterns formed by, for example, an exposure apparatus which is used in semiconductor device manufacturing, as well as to an exposure apparatus which employs the same.
2. Description of the Related Art
Measurements of intervals of the patterns formed by an electron beam, a light beam or X-ray exposure apparatus are essential for highly accurate evaluation of the dimensional distortions brought about by that exposure apparatus and that exposure apparatus's pattern forming process. In, for example, a photo stepper, distortions in a photographing system, such as an optical system for forming a demagnified image of a reticle onto a wafer, or distortions resulting from inaccuracy in the placement of the stage or the wafer which appear in the pattern formed on the wafer. Where a high degree of integration ensuring patterned lines of submicron width is required, such distortions must be measured to an accuracy of 0.01 .mu.m or less, and means for measuring the distance between the patterns to a corresponding accuracy must be provided.
Conventionally, the distance between the patterns is measured by projecting converged rays of light onto pattern P on wafer U, as shown in FIG. 1 (a), and by observing the light scattered from an edge of the pattern P. That is, a converged laser beam La scans the pattern P, which is an object to be measured, formed on the wafer U which is an object to be detected, perpendicular to the pattern P. The light scattered from the edge of the pattern P is detected by a pair of detectors D1 and D2, and line width W or the distance between the patterns P is detected from the scanning distance of the laser beam La, as shown in FIG. 1 (b).
However, such a measuring method causes problems with resist patterns employed in an actual wafer process, because they may not have a clear edge; rather, the contour thereof may have an unclear edge. Consequently, errors of 0.1 to 0.2 .mu.m may be generated between the measured value and the true value. This precludes accurate evaluation of the distortions of an optical printing system by using the dimension between the patterns.